Alexis, who a navy source said was discharged after a series of misconduct issues, was armed with a machine gun and a pistol and there are reports he also had a shotgun.

Killeen, Texas, October 1991:

A man shot dead 22 people in a restaurant and then killed himself.

George Hennard crashed his pick-up truck through the wall of Luby's cafeteria before opening fire. Twenty others were injured in the attack.

Littleton, Colorado, April 1999:

Two teenage boys shot and killed 12 students and a teacher at Columbine High School.

Eric Harris, 18, and Dylan Klebold, 17, were carrying bombs when they opened fire around the school.

They eventually turned their guns on themselves in the school library.

Another 24 students were injured in the attack which is the deadliest mass murder committed on an American high school campus.

In addition to shootings, the complex and highly planned attack involved a fire bomb to divert firefighters, propane tanks converted to bombs placed in the cafeteria, 99 explosive devices, and bombs rigged in cars.

Atlanta, Georgia, July 1999:

A stock market trader went on a shooting rampage, killing 12 people, including his wife and two children, before taking his own life.

It is believed that Mark Orrin Barton was motivated by a $105,000 loss in the lead up to the shootings.

Four hours after his rampage began he took his own life at a petrol station in Acworth after ignoring police orders to stop.

After the tragedy, police discovered the bodies of his family members who had been killed by hammer blows. A note at the scene suggested they had been killed in the days leading up to the shootings.

Red Lake, Minnesota, March 2005:

A teenage boy killed two people at his grandfather's home on an Indian reservation before going to his local high school where he killed seven others.

Jeffrey Weise killed his grandfather (a tribal police officer) and his girlfriend at his residence before driving to Red Lake Senior High School. The dead included a security guard, a teacher and five students.

Weise killed himself in an empty classroom.

Blacksburg, Virginia, April 2007:

A student went on a rampage at Virginia Tech, killing 32 people.

Seung-Hui Cho, 23, a senior at the school went on two separate attacks two hours apart before committing suicide.

The massacre is the deadliest shooting incident by a single gunman in US history.

Before the attack, Cho had been diagnosed with severe anxiety and two years before the shooting had been accused of stalking two female students.

Manchester, Connecticut, August 2010:

A man who complained of racism at the beer warehouse where he worked went to the facility and shot dead eight people.

Omar Sheriff Thornton had signed forms agreeing to his resignation and after having been escorted from the building took two pistols from his lunchbox and opened fire.

Once police had arrived, Thornton had locked himself in an office. From there he called his mother to explain what he had done. He then dialled 911 to say he wished he had killed more people before turning the gun on himself.

Seal Beach, California, October 2011:

A man angry over a child custody ruling opened fire at the hairdressing salon where his ex-wife worked, killing her and seven other people.

Scott Evans Dekraai, 41, entered the salon with three handguns which he reloaded at least once during the rampage.

Dekraai was arrested half a mile from the scene of the crime.

In October 2011 he was charged with eight counts of murder - prosecutors sought the death penalty.

Dekraai entered a plea of not guilty, and while his trial was meant to get underway in March this year, it has been postponed until March 2014.

It was the deadliest mass killing in Orange County history.

Oikos, California, April 2012:

A male nursing student methodically killed seven people at a Christian university.

Goh, stood up in a nursing classroom while class was in session, ordered classmates to line up against the wall, and fired at them.

He was taken into custody after the shooting which went down as the third deadliest university shooting in US history.

Since his arrest Goh has entered a not guilty plea but is yet to go on trial after psychological examinations deemed him unfit.

On January 7 2013 a judge ordered that he be re-examined every 90 days.

Aurora, Colorado, July 2012:

A man killed 12 people when he opened fire at a movie theatre showing a late-night premiere of a Batman film in a suburb of Denver.

Witnesses said Holmes threw smoke bomb-type devices before opening fire in the cinema with weapons including an AR-15 military-style rifle, a 12-gauge shotgun and a .40-caliber pistol.

James Holmes is also accused of wounding another 70 people and earlier this year pleaded not guilty due to insanity.

The 23-year-old is charged with 166 counts of murder and attempted murder and prosecutors are seeking the death penalty as a punishment.